Existing overload protectors in hermetic compressors use a combination of a heating element and a bimetallic switching device to break the motor circuit if current or temperature limits are exceeded. In a locked rotor condition a current trip will be primarily driven by I2R losses through the heater, while a running high temperature trip is more influenced by the ambient temperature. With either influence, the temperature reaches a point such that the bimetallic switching device xe2x80x9csnapsxe2x80x9d into a new position, thus breaking the circuit.
Typical devices used in motor applications for temperature and/or current protection include those as disclosed in U.S. Pat. Nos. 3,167,699 (Renaud), U.S. Pat. No. 5,615,072 (Hofsass et al), U.S. Pat. No. 5,575,229 (Takeda), and U.S. Pat. No. 4,866,408 (Petraitis et al) and commercially available hermetic motor protectors from Texas Instruments, Inc. of Dallas, Tex. given product designation 33HM800 or the like.
The heaters in the above mentioned devices connect in series with the mains circuit of a motor. When the current in the mains circuit increases to a level above the specified maximum current limit for the motor, the heat generated by this level of current through the heater will be adequate to trip the bimetallic switching device and open the mains circuit.
The present invention provides for the use of a control element powered and/or controlled from a separate control circuit. In one embodiment heating.the bimetallic switching device with this control element, the circuit can be broken on command from a control signal, independent of the mains power conditions. In another embodiment a control element and a normally closed switch are connected in parallel with the mains power circuit. The switch is operated by a control signal. When the switch is closed and the mains circuit is powered, the mains circuit will be broken. Unlike prior motor protectors, which include heaters not independently controlled, the present invention incorporates at least one heater which is operatively connected to a control circuit for controlled actuation of the protective device.
One advantage of the present invention is that the circuit can be broken from a control signal, independent of the mains power condition. The control signal will allow the circuit to be broken due to other factors or conditions other than the exceeding of current or temperature limits.
Another advantage of the present invention is that with the inclusion of current and temperature monitoring devices into the control circuit, the heating element for the current protection in the overload protector could be eliminated and the overload protector would not have to be located in such an extreme environment to detect the temperature trip level.
Another advantage of the present invention is that it does not require a large heat sink as would a silicon rectifier.
Another advantage of the present invention is that power must be applied to the control circuit and no trip conditions sensed before the motor can be operated.